1. Field of the Invention
This invention relates to a novel mannose isomerase which catalyses the mutual conversion reaction between D-mannose (hereinafter referred as mannose) and D-fructose (hereinafter referred as fructose), a process for producing it and a process for producing mannose from fructose using it.
2. Related Background Art
It has been recently shown that mannose inhibits the growth of harmful enterobacteria Salmonella [R. H. Brown, Foodstuff, Jun. 12, 10 (1989)], indicating its utilization as feed additives for fowls such as chickens, or as bioactive food materials. However, mannose is very expensive because it is ordinarily prepared by hydrolysing mannan contained in vegetables such as woods or konjak (devil's tongue).
The present inventors have been seeking after microorganisms being capable of producing industrially usable mannose isomerase which converts fructose to mannose reversibly, in order to establish a technique to produce mannose from fructose which is produced abundantly and inexpensively, as well as a technique for preparing mannose directly from glucose with the combination use of mannose isomerase and glucose isomerase which converts glucose to fructose reversibly. Consequently, a bacterium was isolated which has been identified as one strain of the genus Pseudomonas producing a mannose isomerase markedly excellent in thermal stability as compared with those known in the prior art.
Mannose isomerase was found by Palleroni and Doudoroff et al in 1956 as the first enzyme of the isomerizing enzymes for free hexoses in Pseudomonas saccharophila [J. Biol. Chem., Vol. 218, p. 535 (1956)]. Later, the present inventors found that a bacterial strain isolated from soil and identified as Xanthomonas ruburilineans produced a mannose isomerase [Journal of Agricultural Chemical Society of Japan, Vol. 37, P. 524-528 (1963), Agric. Biol. Chem., Vol. 28, p. 601-604 (1964)], and also presence of a similar enzyme in Streptomyces aerocolorigenes [Annual Report of Fermentation Institute, Agency of Industrial Science & Technology. Vol. 28. p. 89-94 (1966)]. However, all of these enzymes have optimum temperatures at 35.degree. to 40.degree. C. and are poor in thermal stability, and therefore cannot be utilized in industry.